Giving animals antibiotics may make them sicker and could lead some to spread even more salmonella than they would have otherwise, U.S. researchers experimenting on mice said.

The findings in the Proceedings of the National Academy of Sciences could point to a new concern over feeding healthy livestock low doses of antibiotics to help them grow and stave off common illnesses, a practice that critics say may fuel drug-resistant superbugs.

Researchers at Stanford University School of Medicine gave oral antibiotics to mice infected with Salmonella typhimurium, a bacteria which can cause food poisoning.

A small minority, known as “superspreaders” because they had been shedding high amounts of salmonella in their feces for weeks, remained healthy. It appears neither the antibiotic or the illness had much effect on them.

“The rest of the mice got sicker instead of better and, oddly, started shedding like superspreaders,” the university said in a statement describing the research.

A previous Stanford study found that giving non-superspreader mice an oral antibiotic led to a rapid increase in salmonella shed in their feces.

This study showed that giving streptomycin, an antibiotic, to salmonella-infected mice, led most of them to begin shedding high levels of the pathogen in both their gut and their feces.

Most of the treated mice also appeared sicker after the antibiotics.

“They lost weight, had ruffled fur and hunched up the in corners of their cages,” said Denise Monack, associate professor of microbiology and immunology and the study’s senior author.

“They also began to shed much larger quantities of bacteria.”

The same thing happened when the mice were given another antibiotic, neomycin, suggesting that the medicine had the opposite of its intended effect.

“If this holds true for livestock as well — and I think it will — it would have obvious public health implications,” Monack said.

“We need to think about the possibility that we’re not only selecting for antibiotic-resistant microbes, but also impairing the health of our livestock and increasing the spread of contagious pathogens among them and us.”